Solution Everyone Is Listening No One Is Transmitting

Never the least stir made the listeners.

Walter de la Mare, The Listeners

We have briefly discussed the difficulties in trying to receive a signal from ETCs. We have not considered how difficult it might be for them to send a signal. One thing seems certain: no matter how hard it is to detect a signal from an unspecified planetary system among the Galaxy's hundreds of billions of stars, it must be a lot harder to send it — at least, to send it with any expectation that it will be detected. Could it be everyone is listening and no one is transmitting?

in a sense, our civilization already transmits signals to the heavens. For several decades, our radio and TV transmitters have been leaking EM radiation into space. As i write, live broadcasts about the fall of the Berlin Wall could be sweeping across the star Tau Ceti; news of Kennedy's assassination could now be reaching Arcturus; cricket lovers in the Castor system may soon receive word of Bradman's last Test innings. But it is debatable whether these transmissions will be detected, even if ETCs are listening. Our transmitters direct their beams horizontally, to be picked up by individual antennae. so although some of the output is lost to space — a beam of EM radiation sweeps across space as Earth rotates on its axis and as it orbits the sun — it is down to luck whether any of it intersects with a distant star. Furthermore, the high bandwidth and relatively low power of our transmitters mean even an Arecibo-type telescope would struggle to detect our broadcasts much beyond the orbit of Pluto. so unless ETCs are nearby, extremely lucky, and have a level of receiving technology far beyond our own, they are unlikely to detect our inadvertent transmissions.133 Besides, the amount of this leakage radiation is lessening as we increase our use of cable. (The radiation from powerful military radars, and the signals that astronomers bounce off Venus and Mars to map the topography of those planets, has more chance of being detected over interstellar distances. On the other hand, such radiation is highly focused; the beam is unlikely to intersect with an alien receiver.)

What if we wanted to be noticed? Rather than trusting to luck and hoping an ETC spots our TV (hoping too, perhaps, they receive Cheers rather than Charlie's Angels), we would need a means of transmitting a powerful narrowband signal. This is the flip side of SETI: instead of pondering how best to listen we consider the practicalities of how to transmit. Of course, by studying the problem of how to transmit a signal over interstellar distances, we can learn a lot that will help us to listen for signals.

Suppose we decide to use radio. The first problem is which transmission frequency to use. The logic that makes us listen for signals at the waterhole suggests we should transmit somewhere in that region, although arguments could be made for several other frequencies. Once we have decided on a frequency — and let us assume for the moment that we should broadcast in the waterhole — what technology would be required?

Since we do not know in advance where an ETC might reside, the safest option is to transmit isotropically — with the same power in all directions. If we wanted to send a narrowband signal so that it could be detected by a small antenna at a distance of 100 light years, say, then the power required by the transmitter would exceed the present total installed electricity-generating capacity of the world. And 100 light years barely extends beyond our immediate neighborhood. The farther away we want the signal to be received, the larger the power requirement of the transmitter. An isotropic transmitter is thus quite beyond our present technological capability. Even if we could build such a device, would we commit such a large level of resource to a project that has no guarantee of success?

If ETCs listen with an Arecibo-type telescope rather than a simple dish, then the power requirements for the transmitter lessen. Indeed, if we knew the precise location of an Arecibo-type telescope on the other side of the Galaxy, then our own Arecibo could send it a signal. The problem is that we do not know in advance where to point the transmitter. An Arecibo-type dish, operating at a frequency in the waterhole region, has an extremely narrow beam. The old needle-haystack dictum does not begin to convey the improbability of sending a narrow beam that just happens to align with a large receiver somewhere in the depths of space.

Isotropic transmission, which guarantees that anyone with an ear can hear you, is exceedingly expensive; beamed transmission, which is cheap, excludes most of your potential audience. These are the two extremes for a transmission strategy. We could make various trade-offs and compromises, and ETCs may be able to devote more resources to transmission than can mankind at present. But interstellar radio transmission is not easy.

In the light of these difficulties — and there are several others I have not described — maybe ETCs decide to let others do the hard work of transmission. Maybe the Galaxy is full of civilizations waiting for a call?

This is an unlikely resolution of the paradox. The difficulties may seem insurmountable to us, but they would surely present less of a challenge for, say, a K3 civilization. And many of the problems surrounding transmission are surmountable even with our present level of technology — if we move away from the idea of using radio waves!

Even with our present laser technology we can generate a pulse of light that, for a short duration, outshines the Sun. An advanced ETC would presumably have no trouble in generating a pulse that is, briefly, billions of times brighter than its star. Such pulses can be detected with a relatively small optical telescope connected to a charge-coupled device. Furthermore, over distances of a few thousand light years, the interstellar medium has relatively little effect on a visible light signal; unlike radio, optical communication is not corrupted. Lasers are in many ways a more effective transmission mechanism than are radio dishes.

The drawback with optical-based communication is that the beam is extremely narrow. The transmitting civilization must therefore know the precise location of the receiving telescope. It is the same problem radio transmitters face if they generate narrow-beam signals, except it is much worse. It is futile to send a laser signal at random; the beam is unlikely ever to be detected. The transmitting civilization must therefore draw up a list of target planetary systems along with precise and accurate values for the positions of those systems. Furthermore, stars are not at rest. If an ETC sends a signal to where the star is now, then by the time the light reaches it the star will have moved on. So the transmitting civilization also needs accurate information about the velocities of the target stars.

Gathering information about other planetary systems and the precise location and velocity of stars is not easy; but neither is it impossible for a civilization advanced beyond our own. The recent Hipparcos mission obtained such data on the nearest stars, and proposed projects like the ESA Darwin mission and NASA's Terrestrial Planet Finder will detect any Earth-sized planets around the nearest 200 stars.134 If we can contemplate such missions, then a civilization just a little more advanced than ours should be able to use optical communication over interstellar distances — and radio signals too, if they choose. So there seems to be no technical reason why ETCs cannot transmit.

It is worth mentioning that mankind has already beamed two signals to the stars (deliberately, that is, as opposed to leakage from broadcasting stations). The first intentional signal was sent in 1974.135 Its author was Drake, who took the opportunity to use the inaugural ceremony of the refurbished Arecibo telescope to send a message at 2.38 GHz in the direction of M13. (This is a globular cluster containing about 300,000 stars, but unfortunately not of the type we expect to possess Earth-like planets.) The message lasted 3 minutes and was only 1679 bits long, but Drake managed to pack in a lot of information. When the signal reaches M13 in about 24,000 years time, if astronomers there could decode it, they might learn a surprising amount about us. Even if they could not decode it, the very detection of the signal would convey information; it would tell them an intelligent species was here and it had advanced to the radio stage — the very fact of the signal carries a message. (The second broadcast, in 1999, was a 400,000bit message at 5 GHz to four nearby Sun-like stars. The message was sent several times; unfortunately, the first transmission contained a typographical error.)136

It is also worth mentioning that Drake was criticized because he made his broadcast without consulting widely. The transmission represented Earth, yet no national governments were asked their opinion about the content of the signal. In practice, isolated transmissions like this have essentially no chance of being detected; but perhaps future large-scale transmissions from Earth will require a planetary government that can speak for us all. Perhaps an advanced ETC, recognizing the ethical problems of transmitting signals to the Universe, only transmits when it has achieved a level of unity such that its signals represent a consensus of their entire world. And perhaps that is why we are still waiting to hear from them: they listen not because of technical difficulties but because of ethical difficulties.137

This is another unlikely resolution to the paradox. Attributing motives to putative alien civilizations is probably futile. And once again we have to ask whether a concern over the ethical niceties of transmission would affect every civilization? All we can say with certainty is that sending a message to the Universe, in the expectation that it will be received by another civilization, is difficult. But it is not impossible. Some civilizations should be out there, signaling their presence. So why have we not heard from them?

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